This word means "having no like or equal" and expresses absoluteness, as do words such as round and square. Logically, therefore, the word unique cannot be compared; something cannot be "more unique," "less unique," "more round," "less round." If a qualifying word such as nearly is used, the illogicality is removed."This is the most unique painting in the museum" is not standard, but "This is the most nearly unique painting in the museum" is. Unique is not only loosely used but overused; for it, certain words can be substituted on occasion such as novel, exceptional, remarkable, rare, inimitable, peerless, incomparable, uncommon, and unusual. See also incomparable.
Dictionary of problem words and expressions. Harry Shaw. 1975.